Unnecessary edit. Sergio is long past his prime, but will always be world class, just like David Silva, Yaya Toure, Vincent Kompany, Kevin De Bruyne…
Yes, this season has been a bridge too far to include them all in the same team at once, as I discussed regarding the midfield against United. However, Nico was never going to be the person to replace Kova (Gonzales) or Gundo (Grealish) or Bernardo (Doku/Savinho) or even KDB (McAteer) in that team.
He plays because he can fill in at LB. The end. For now.